1. Field of the Invention
The present invention relates to a small-sized direct current rotary electric appliance, and, more particularly, it is concerned with an improved structure of the brush therefor.
2. Description of the Prior Arts
A recent trend in the field of small-sized direct current rotary electric appliances is the demand for such appliances having a flattened configuration, wherein the axial length thereof is made as short as possible.
Such flat rotary appliance with shortened axial length is mainly utilized as a motor for audio equipment, e.g., as a motor for stereo-phonic equipment, etc. in automobiles. Since the recent oil crisis, the world has been demanding, as a general tendency, more compact cars for saving the limited natural resource, and also, from the standpoint of designing cars adapted for a safer driving norm, various gauges and instruments installed in dash-board of the automobiles which appeal to the sense of sight are required to be given more space than ever before. In the latter event, however, when a larger space is provided within a certain limited area of the dash-board for such instruments appealing to the eyesight, the space for stereo-phonic equipment, etc. which utilize the sense of hearing would naturally be sacrificed. Under such circumstances, the attempt to flatten rotary electric appliances like motors in their axial directions is highly significant in view of miniaturization and flattening of the audio equipment, particularly, for automobiles.
As a method for realizing such flat-type appliance using the armature as the rotor, there has heretofore been practiced a way, in which the commutator is arranged along the axial direction of rotor shaft. A step further, there has also been developed such flattened rotary electric appliance, as seen in U.S. Pat. No. 2,818,518, FIG. 4, for example, wherein the commutator in a disc-shape is fixed on the rotor shaft substantially perpendicularly thereto so that the brushes may be in contact with the surface of the commutator in a direction substantially parallel thereto, thereby reducing the axial length of the commutator.
The motor disclosed in the U.S. Pat. No. 2,818,518 is in such a construction that both ends of the rotor shaft are rotatably supported on the motor casing through bearings, and a lubricant has to be supplied between the rotor shaft and the bearings in order to reduce the friction therebetween and facilitate rotation of the rotor shaft. However, in the case of a motor as disclosed in U.S. Pat. No. 2,818,518, wherein the armature is used as the rotor, when the thus supplied lubricant flows along the rotor shaft and penetrates into the armature windings, the armature windings are heated to cause eventual damage or other mal-effect thereto. In such motor, therefore, it is necessary to provide an oil thrower, or slinger, on the rotor shaft at both sides of the armature winding. However, in the motor of a type, wherein brushes in the form of a cantilever spring are caused to contact on the surface of the commutator along a direction substantially parallel thereto as taught in U.S. Pat. No. 2,818,518, when the oil throwers are to be provided, the member came into contact with the brushes to hinder assembly of the motor. Moreover, such oil throwers, if provided exclusively for the purpose of slinging the lubricant oil, inevitably increase the axial length of the rotor for the axial length of such members, which is therefore not desirable for realizing flattening of the motor.
Besides the abovementioned U.S. Pat. No. 2,818,518, a motor, in which a disc-shaped commutator is fixed on the rotor shaft in a manner to be substantially perpendicular thereto, and the brushes are caused to contact the surface of the commutator along a direction substantially parallel thereto, is also disclosed in British Pat. No. 1,111,527. In such motor, the contact point between the brushes and the commutator has to be brought to as close a position as possible with respect to the axis of rotation so as to minimize the loss of torque resulting from the friction between the brushes and the commutator, due to which requirement the brushes are limited in their width and are formed in an elongated shape. Consequently, the brush disclosed in U.S. Pat. No. 2,818,518 (FIG. 4) is formed as a very thin and long rod, one end of which is fixed and the other end of which is free to serve as a contacting end to the commutator. Such brush is accompanied by various defects in that it undergoes vibrations with the position at which it is fixed being made the fulcrum thereof and, since the distance from the fulcrum to the free contacting end thereof is long, the brush vibrates with a very large amplitude. When the motor is rotated, stable contact of the brushes with the commutator is hindered, thereby causing not only undesirable noise but also detachment of the brush from the commutator with eventual interruption of electric conduction. In order to avoid the drawbacks of the brush as disclosed in U.S. Pat. No. 2,818,518, the British Pat. No. 1,111,527 provides a brush of simply increased width, one end of which is fixed, and the other end of which is formed in an extremely thin plate spring and made free to serve as the contacting end with the commutator. Even in this case, however, the brush as a whole undergoes vibrations with the position at which it is fixed, being made the fulcrum and, since the distance from the fulcrum to the free contacting end is long, the brush vibrates with a very large amplitude when the motor is rotated, thus hindering stable contact of the brush with the commutator. Therefore, not only undesirable noise but also detachment of brush from the commutator with eventual interruption of electric conduction is caused. Thus, the simple increase in thickness or width of the brush does by no means constitute a satisfactory solution to the abovementioned drawbacks.
Further, in order to alleviate the abovementioned drawbacks, there has also been known already a brush structure, as disclosed in Japanese Patent Publication No. 39-5905, wherein a plate spring of a shape having relatively similar longitudinal and transversal lengths which may be circular or square is provided with a fixing hole in the center part thereof, and in the outer periphery of which a U-shape or a right-angled U-shape opening or window is formed to construct elastic arm portions on both sides of the fixing hole in a symmetrical manner. Two contact pieces which contact with the commutator are integrally formed outside of the center part between the two elastic arm portions to increase the width of the brush, and to reduce length of the brush. Furthermore, DAS No. 1,293,318 discloses a similar brush structure consisting of a plate spring of a shape having relatively similar longitudinal and transversal lengthes either circular or a square and provided with a fixing hole in the center part thereof, in the outer periphery of which a U-shape, or a right-angled U-shape opening or window is formed therearound to constract elastic arm portions on both side of the fixing hole in a symmetrical manner. Two contact pieces contacting with the commutator are formed integrally between the two elastic arm portions, and at an outside position where it is nearer to one of the arm portions to thereby widen the width of the brush and to shorten its length.
However, such brushes as disclosed in Japanese Patent Publication No. 39-5905 and DAS No. 1,293,318 undergo a concentrated bending moment at the connecting portion between the center part provided with the fitting hole and the elastic arm portions, i.e., at the extreme end of the U-shape, or right-angled U-shape opening or window, as a result of which they bend at the connecting part which acts as the fulcrum, while tending to be twisted. Consequently, such brushes are unable to exhibit spring characteristics to a satisfactory and effective degree, and also lack stability when they contact the commutator.